1. Field of the Invention
This invention relates to pneumatic tires having their belts reinforced with steel cords.
2. Prior Art
Steel wire cords are known to be superior to cords of other materials in respect of physical strength and hence find extensive application inter alia as tire belt reinforcing elements particularly for the tread portion which is subject to the most severe load burden. Steel cords however are disadvantageously large in specific gravity, leading to increased weight of product tires counter to the automobile industry policy of providing lighter cars to reduce the resistance of rolling along the roadway and hence to save fuel consumption.
Flexibility or suppleness of the tire constituent parts is an important factor to cope with sagging behavior of the tire while rolling on the roadway. Steel cords in the belts of the tire may be theoretically preferably formed from as many steel filaments twisted together as is feasible to provide increased flexibility. This will however result in increased empty spaces or voids formed in the strands of the filaments extending longitudinally along the axis of the cord, and consequently air and moisture penetrate into the voids, causing corrosion or rust of the cord. With most advanced automobile tires, therefore, there are used a minimal number of two steel filaments twisted together.
Steel cord strength would decline if the total cross sectional surface area of the cord is reduced in an attempt to reduce the tire weight. With this in view, it has been proposed to use a steel cord consisting of two intertwisted filaments of a high tension steel having high carbon contents as disclosed for example in Japanese Laid-Open Patent Publication No. 62-117893, in which instance the amount of the rubber used to coat or embed the cords therein is increased so as to restrain separation between the coating rubber and the cords notably at the marginal ends of the belts as cross-sectionally observed. However, the higher the modulus of the coat rubber, the less flexibility is available for the belts, rendering the cords susceptible to breakage or rupture. On the other hand, increasing the weight of the coat rubber will literally increase the tire weight and its rolling resistance.